We are creating computer models of early C. elegans embryos to identify blastomeres that contact one another and are therefore candidates for juxtacrine cell signaling events early in animal development. Studies of embryogenesis provide evidence that specific cell-cell contacts are required for the correct induction of pharyngeal, neuronal, and hypodermal tissues (Hutter and Schnabel, Development 120:(7):2051-2064, 1994; Development 121(5):1559-1568, 1995; Moskowitz et al., Development 120(11):3325-3338, 1994). We are using images of 12-blastomere stage embryos obtained by high voltage electron microscopy of 0.25 fm sections and time lapse light microscopy with Nomarski optics to generate three dimensional computer models. Time-lapse Nomarski microscopy permits the examination of larger numbers of animals and the unambiguous identification of each blastomere. Electron microscopy enables us to locate accurately the cellular membranes in the embryo. We have perfected a cryofixation and freeze-substitution methodology for small populations of well-synchronized embryos, which are then flat-embedded and optimally oriented for thick-section HVEM. This cryofixation technique preserves the fine structure of cell membranes, while leaving the embryo's shell intact to minimize the rearrangement of blastomeres during preparation. In addition to elucidating cell-cell contacts during embryogenesis, the computer models provide information about approximate cellular volume and the surface area of each cell-cell contact. Recent modifications of the IMOD software package allow the user to track lineage information and model cell positions and movements throughout embryogenesis. The software is also ready to be used in routine lineage analysis of C. elegans mutant animals and to model cellular movements during gastrulation. C6